This application relates generally to electrical power generation by an aircraft, and more specifically to a pole count changing electric generator.
A typical turbofan gas turbine engine for an aircraft has a high-pressure (HP) spool shaft and a low-pressure (LP) spool shaft. The LP spool shaft is turned by the low pressure turbine of an aircraft turbofan engine and typically operates across a broader range of rotational speeds. The HP spool shaft typically operates within a narrower range of rotational speeds. It is additionally known that coupling a rotating shaft (such as the HP or the LP spool shaft) from a turbofan engine to an electric generator can be an efficient source of electrical power generation on an aircraft.
Some vehicles incorporating gas turbine engines, such as aircraft, require significant amounts of electric power for operation. In some aircraft applications, a typical generator driven at the typical speed range of a HP spool shaft would produce electric current within a frequency range of 360-800 Hz. This frequency range is acceptable. Therefore, the high-pressure (HP) spool shaft of a turbine engine is typically used to generate electricity for an aircraft system. However, in some aircraft applications the demand for electric power exceeds the power extraction potential of the HP spool shaft.
The rotational speed of an LP spool shaft varies over a much broader range, typically encompassing rotational speeds having a speed ratio of 4.44:1. Although power extraction from the LP spool shaft is possible, the broader range of rotational speeds of the LP spool shaft would produce current whose frequency exceeds the 360-800 Hz range at higher speeds.
If power were delivered to the onboard electronics at too high of a frequency it could damage the onboard electronics. Due to the wider shaft speed range of the LP spool shaft, generators using the current state of the art cannot consistently generate electrical power from the LP shaft rotation at a usable frequency.
Electrical generators contain a number of rotor poles. The number of rotor poles combined with the speed of the rotor determines the output frequency of the electrical power from the generator. A potential solution to the frequency range problem described above is illustrated in pending U.S. patent application Ser. No. 11/645,438 (the '438 application) “Pole Shifting Generator” invented by the inventor of this application. The '438 application discloses a generator that utilizes a transistor network to shift the polarity of certain pole pairs thereby altering the number of magnetic poles an electric generator sees without reducing the number of poles through which current travels, and consequently reducing the output frequency.
Despite the difficulty of consistently generating usable electric power from the LP spool shaft, it is highly desirable to utilize the LP spool shaft's rotation to generate usable electric power. It is therefore desirable to design a generator that can provide output power at a usable frequency across the entire speed range of the LP spool shaft rotation.